There have been fabricated prototypes of semiconductor laser devices that oscillate in a region ranging from ultraviolet to visible light by the use of a nitride semiconductor material as exemplified by GaN, AlN, InN, and composite crystals thereof. For such purposes, GaN substrates are typically used, and therefore they have been intensively researched by a host of research-and-development institutions. At the moment, however, no semiconductor laser devices offer satisfactorily long useful lives, and accordingly what is most expected in them is longer useful lives. It is known that the useful life of a semiconductor laser device strongly depends on the density of defects (in the present specification, defects refer to, for example, vacancies, interstitial atoms, and dislocations in a crystal) that are present in a GaN substrate from the beginning. The problem here is that substrates with low defect density, however effective they may be believed to be in achieving longer useful lives, are difficult to obtain, and therefore researches have been eagerly done to achieve as much reduction in defect density as possible.
For example, Non-Patent Reference 1 reports fabricating a GaN substrate by the following procedure. First, on a sapphire substrate, a 2.0 μm thick primer GaN layer is grown by MOCVD (metalorganic chemical vapor deposition). Then, on top of this, a 0.1 μm thick SiO2 mask pattern having regular stripe-shaped openings is formed. Then, further on top, a 20 μm thick GaN layer is formed again by MOCVD. Now, a wafer is obtained. This technology is called ELOG (epitaxially lateral overgrown), which exploits lateral growth to reduce defects.
Further on top, a 200 μm thick GaN layer is formed by HVPE (hydride vapor phase epitaxy), and then the sapphire substrate serving as a primer layer is removed. In this way, a 150 μm thick GaN substrate is produced. Next, the surface of the obtained GaN substrate is ground to be flat. The thus obtained substrate includes, within a substrate surface, a defect-concentrated region and a low-defect region, and, in general, it is classified into a defect-concentrated region including many defects in a part of SiO2 and a low-defect region being all the remaining part of SiO2.
The problems here is, however, that the characteristics of a semiconductor laser device fabricated by growing a nitride semiconductor layer, by a growing process such as MOCVD, on a substrate including a defect-concentrated region and low-defect region vary, resulting in a remarkably low yield rate.
As a result of an intensive research on why the characteristics of a semiconductor laser device fabricated by growing a nitride semiconductor layer, by a growing process such as MOCVD, on a substrate including a defect-concentrated region and low-defect region vary, resulting in a remarkably low yield rate, the applicant of the present invention has found out that this is because poor flatness of the film surface results in poor surface morphology. Specifically, when a nitride semiconductor layer (particularly, an InGaN layer used as an active layer) is grown on an irregular surface of the film, the thickness and composition of the layer vary depending on the surface irregularities of the film, and thus greatly deviate from the set values. Furthermore, the applicant has found out that the poor surface morphology greatly depends on the shape of the defect-concentrated region in the nitride semiconductor layer. That is, the applicant has found out that the growth direction and mode of a thin film strongly depends on the shape of the defect-concentrated region, and therefore the irregularly-shaped defect-concentrated region degrades the flatness of the film surface, leading to poor surface morphology. Growing a thin film such as an active layer on such an irregular surface causes the device characteristics to vary.
These results are obtained in experiments conducted in the following manner. First, a case where a nitride semiconductor layer is grown on a substrate including a defect-concentrated region and a low-defect region will be described. FIG. 16(a) is a sectional view of a conventional semiconductor laser device, and FIG. 16(b) is a top view of FIG. 16A. Reference numeral 10 represents a substrate including a defect-concentrated region and a low-defect region, reference numeral 11 represents a defect-concentrated region, reference numeral 12 represents a low-defect region, reference numeral 13 represents a nitride semiconductor layer, and reference numeral 13a represents a surface of the nitride semiconductor layer.
If a nitride semiconductor layer is grown directly on the substrate 10 (i.e., without performing any preliminary treatment for the substrate, etc.), the growth rate of the defect-concentrated region is greatly different from that of the low-defect region, because the defect-concentrated region has lower crystallinity than the low-defect region and may have a growth surface that does not appear in the low-defect region. As a result, the defect-concentrated region grows at a lower growth rate than the low-defect region, and thus growth hardly occurs in the defect-concentrated region.
FIG. 17(a) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of lines grows, and FIG. 17(b) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of dots grows. In either case, since growth hardly occurs in the defect-concentrated regions, growth is started at the defect-concentrated region x and proceeds in the direction indicated by arrow A, and growth is started at the defect-concentrated region y and proceeds in the direction indicated by arrow B. When growth occurs in two different directions in this way, the layer thickness in a growth meet portion becomes different from that elsewhere, leading to poor surface flatness.
FIG. 18 shows measurements of the roughness as measured in the direction [11-20] perpendicular to the line-shaped defect-concentrated region and in the direction [1-100] parallel thereto. The measurements were made by using the “DEKTAK3ST” model manufactured by A SUBSIDIARY OF VEECO INSTRUMENTS INC. The measurement was conducted under the following conditions: measurement length: 600 μm; measurement time: 3 s; probe pressure: 30 mg; and horizontal resolution: 1 μm/sample. The level difference between the highest and lowest parts, within the 600 μm wide region in which the measurement was taken, was found to be 200 nm. Here, the large grooves in the defect-concentrated regions are not considered.
In addition, the growth meet portion was found to be a non-luminous region. Thus, it can be said that the difference in thickness between the layers within a wafer surface causes the device characteristics to vary.